In a photolithography process for manufacturing semiconductor devices and liquid crystal displays (LCD's), resist is coated on a substrate, and the resultant resist coating film is exposed to light and developed. The series of processing stages are carried out in a coating/developing system having discrete heating sections, such as a prebaking unit and a postbaking unit. Each heating section incorporates a hotplate with a built-in heater of a resistance heating type.
Feature sizes of semiconductor device circuits have been reduced to less than 0.1 microns. Typically, the pattern wiring that interconnects individual device circuits is formed with sub-micron line widths. To provide reproducible and accurate feature sizes and line widths, it has been strongly desired to control the heat treatment temperature of the photoresist film more accurately. The substrates or wafers (i.e., objects to be treated) are usually treated or processed under the same recipe (i.e., individual treatment program) in units (i.e., lots) each consisting of, for example, twenty-five wafers. Individual recipes define heat treatment conditions under which prebaking and postbaking are performed. Wafers belonging to the same lot are heated under the same conditions.
According to each of the recipes, the heat treatment temperature may be varied within such an acceptable range that the temperature will not have an effect on the final semiconductor device. In other words, a desired temperature may differ from a heat treatment temperature in practice. When the wafer is treated with heat beyond the acceptable temperature range, a desired photoresist film cannot be obtained. Therefore, to obtain the desired photoresist film, a temperature sensor is used for detecting the temperature of the hotplate. On the basis of the detected temperature, the power supply to the heater may be controlled in a feedback manner. Because the temperature of the entire hotplate is not uniform and varies with time, it is difficult to determine the temperature of the hotplate by a single temperature sensor at any instant in time. In addition, the wafer curvature affects the temperature uniformity.
Post exposure bake (PEB) plays an important role in the photoresist process. Heat-treating a resist may have many purposes, from removing the solvent to catalyzing the chemical amplification. In addition to the intended results, heat-treating may cause numerous problems. For example, the light sensitive component of the resist may decompose at temperatures typically used to remove the solvent, which is an extremely serious concern for a chemically amplified resist since the remaining solvent content has a strong impact on the diffusion and amplification rates. Also, heat-treating can affect the dissolution properties of the resist and thus have direct influence on the developed resist profile.
Chemically amplified resists (CAR's) were developed because of the low spectral energy of DUV (deep ultraviolet) radiation. CAR's are temperature sensitive and are used for enhancing the exposure process. A CAR comprises one or more components, such as chemical protectors, that are insoluble in the developer and other components, such as photoacid generators (PAG's). During an exposure step, the PAG's produce acid molecules that include the image information. Desirably, the acid molecules remain inactive until a PEB is performed. The PEB drives a deprotection reaction forward in which the thermal energy causes the acid to react with the chemical protectors. CAR's are particularly sensitive to temperature variations during heat treatment and wafer curvature can affect the temperature variations.
What is needed, therefore, is a method for determining the curvature of a substrate/wafer to more accurately control the temperature variations in a thermal processing system.